Here is the problem: After manually pull the autofocus ring to the infinite, then the autofocus seems can "hunt" backwards;

But it just won't "hunt" forwards, just gave a faint click sound.

I opened it and found out it was the broken flex-cable. The cable has 6 thin wires included. It connects the status of the focus ring (The brush scans the 6 programmed lines on the inside wall of the focus ring to know where it is) to the main circuit board. It is a long cable to be connected to the main circuit board. It has also to be bent in order to be fitted to the flexible focus distance (zoom). After many times of bending and stretching, it breaks apart. Then I fixed it by manually soldering the 6 wires. See my youtube step by step video: